Silver halide emulsions containing silver halide grains wherein a grain of an initial halide composition is formed and then overcoated with one or more other halides are known to the art. Such emulsions are disclosed in the patents set forth below.
U.S. Pat. No. 3,317,322 discloses silver halide grains having high internal sensitivity; said grains comprising a central core of chemically sensitized silver halide and an outer shell of silver halide which is chemically sensitized subsequent to the formation of the outer shell.
U.S. Pat. No. 3,206,313 discloses silver halide grains having high internal sensitivity; said grains comprising a central core of chemically sensitized silver halide and an outer shell of silver halide wherein the outer shell may or may not be chemically sensitized.
British Pat. No. 1,027,146, published Apr. 27, 1966, is directed to a method for forming direct positive emulsions comprising composite silver halide grains having a narrow particle size distribution composed of a core and one or more contiguous layers which differ from each other in composition. The emulsions are prepared by forming silver bromide nuclei by a double jet method which comprises adding silver nitrate and an alkali metal halide to a solution of gelatin at a pAg between 6 and 10. The surface of the thus-formed nuclei grains is then sensitized by physical or chemical modification of the grain surface. The thus-sensitized nuclei are then provided with one or more layers of silver halide by carrying out silver halide precipitation at a pAg of about 9 or more. Although the examples only show silver bromide as the siliver halide nuclei it is also stated that other silver halides, such as silver chloride could also be employed.
The initially formed grains can also be modified by the method set forth in U.S. Pat. No. 3,622,318, for example, which discloses so-called "converted halide silver grains" which identify silver halide grains formed at least partly of a silver salt more soluble than silver bromide and converting at least a portion of the grains to silver bromide or silver iodobromide.